Please Help: What is correct length of trenches required for horizontal slinky loops?

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Please Help: What is correct length of trenches required for horizontal slinky loops?

I am constructing a new house in central NJ and have contacted 3 HVAC geothermal contractors for sizing and bids. The estimates for trench & pipe length I have gotten are so varied that I am having a real quandary in deciding what to do. I am a person who does not know anything about HVAC system. The spec for the house is given below

- Contractor #1 recommends a 5 Ton systems with about 125’ of trench per ton with slinky loop, with 36” diameter and 10” pitch – he was just speaking from experience.
- Contractor #2 recommends a 7 Ton system with about 125’ of trench per ton with slinky loop, with 36” diameter and 10” pitch – he was just speaking from experience.
- Contractor #3 actually showed me a manual J he did and he says he used Wrightsoft hvac software to come up with loop design. This person has given me most information, but his estimates are enormously higher than others. His load calculations were as follows

As you see the estimates are quite disparate and I not knowing much do not have any idea as to what is a good estimate. But after reading through various geothermal and hvac web sites, it seems that about 100’ – 125’ of trench per ton seems more in line with most what is written and can be found over the web.

If required I could send out the blue prints or any other supporting documents I have. Please, any help is very much appreciated and thank you all in advance.

well, sounds like the 3rd contractor is on the right track, however, the loop design is a little big, to say the least. I live in Ohio, I design for a 140' trench, with 600' of pipe in each trench, and there is 1 trench for each ton. I would ask him to check his loop design again, dont know if NJ is that much colder than ohio, but I use Wrightsoft myself, and it works well. If the heating and cooling loads are correct, i would use the 6 ton myself, but the loop i am sure would be different. Maybe ask him to double check his loop.

Thank you Tech01. You kind of confirm my suspension that the estimates are kind of high. Could he be designing so that I have no auxiliary electric resistance heat? He was telling me that I would need just a 1KW resistance strip for back-up? Or maybe he is designing for the coldest average day here in NJ? Will these design assumptions increase the loop size by so much?

Yes, did the design myself and it looks like he is designing for no resisitancce heat. Personally, I dont ever do that. I cant afford to, my excavation costs would be astronomical, and it would take me out of the ball park. I usually take the heating load and size my resistance for that, and I do this because this is also my Emergency heat, if something should happen to the Geo unit. To make it more efficient, there are two ways you can make it work. Both deal with throttling on the 20KW resistence in stages, so you only get what you need. Resistance heat isnt cheap, so we only use it if its absolutely needed.

1. You can install an outdoor thermostat, which will monitor the outdoor temps, and that is what will throttle on the heat strips.
2. You can install a delay on make timer on the heat strips so that only 10KW will throttle on first, and set the timer for 10 minutes or so that it will come on 10 minutes after the first 10KW.
Either one of those options will make the unit work well, and I have used them both.

How many hrs in the year will be at design conditions (maximum heat/cooling load)? The less it is it the more it makes sense to go for a two stage ac/hp and an additional electrical heater. Have you considered vertical wells?

Thanks Tech01 – thanks for designing it yourself. And it is kind of makes me feel a little more at ease now that I know why the loop design is so large. I think your suggestions are probably in the line that I should be thinking too. I will talk to my contractor and tell him what I have learned here.

Entropie – I do not know the details of the design conditions – but here in central NJ, the lows gets below 10*F a few nights a year. What the contractor had told me was that we would need about 64K BTUH at 2*F outside and we probably get to this temp range not more than 6-7 days in a year, and that too for a few hours during those days. He was planning to use FHP Aquarius EC072-1HZ/VT, I do not know it is two stage, but I had asked him for a two stage compressor unit. Again thanks for your response.

If those manual J numbers are close, I'd say this is a good candidate for a 5-ton unit with a 2 stage compressor. That way in cooling season you will only run on the 1st stage and still get decent dehumidification. Also, just for a point of reference..... I installed a slinky system for my house, but I didn't do separate trenches because it's very sandy here (nothing but cave-ins). I spread 3000' of pipe in an area that measured about 25' by 85'. That was for a 4 ton unit in a smaller house than yours in Michigan. I know there are a lot of differences to your house, but at least you have some idea of how much pipe you're looking at. 32,000 feet of pipe is ridiculous. My gut tells me you'll need somewhere around 3500' of pipe. The more important figure is area. The more area you can spread the pipe in the better. Also, depth. The deeper the better.

I just read this thread. Ask the #3 installer to run the numbers again with a design EWT of 30F degrees. I believe you will see the amount of trench/pipe drop in length and thus the cost of the install. How installs has #3 have under his belt?

Using the BIN data and looking up in IGSHPA's Soil Temperature Variations chart, the estimate minimum soil temperature at 6' would be 45F. With a low ground temp/EWT DelatT (6F), it would require many more feet of pipe to transfer same required amount of BTU's compare to a high ground temp/EWT deltaT (15F).

Bob G: I will ask him to design for 30F EWT. However, I am afraid that I did not understand what you meant by deltaT at 6*F and 15*F – did you mean that I will require more pipe or I will not? The soil that we have at 6 ft deep is damp clay shale.

Entropie: I do not really know how long it stays at that low temp – I would guess maybe a week and that also with a few hours a night at 2*F

If there are just a few days where you have design ambient conditions you could consider to choose a smaller system (lower installation / equipment and operation costs) with an additional electrical heater for the few really cold days. A good installer should give you advise about this possibility.

>>I am afraid that I did not understand what you meant by deltaT
>>at 6*F and 15*F – did you mean that I will require more pipe or
>>I will not? The soil that we have at 6 ft deep is damp clay shale.

The 'DeltaT ' is the temperature difference between two given temperatures. So with the coldest soil temperature at 6 feet being 45F degrees, the EWT design parameter of 39F degrees the delta would be only six degrees. Where as if the EWT design paremeter was set to 30F degrees, the delta would be 15F degrees. The greater deltaT, greater the thermal transfer of BTU's from the ground to the ground loop fluid thus less pipe needs to be buried.

The design parameters and sizing should be to cover 95% of the heating season demands. Installer #3 may have selected 39F degrees either to not use any antifreeze or for an open loop pump/dump.

Just about where in New Jersey are you located? I grew up in Marlboro (by Freehold) as a kid. We had mostly a sandy loam soil there.

Thanks Bob G. Now I understand what you mean by the temp differentials. As Tech01 had pointed out, I think he was designing for 100% of the demand, with almost no resistance backup heating. His recommendation was to go with a 1KW resistance heating strip. We are in Somerset (between Princeton and New Brunswick).